This application is based upon and claims benefit of priority of Japanese Patent Application No. 2001-59370 filed on Mar. 5, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a trochoid gear pump for pumping up fluid such as fuel to be supplied to an internal combustion engine, and more particularly to such a trochoid gear pump in which imbalance pressure generated therein is canceled.
2. Description of Related Art
Recently, a study to use a trochoid gear pump as a fuel pump for supplying fuel into an internal combustion engine is being made in order to improve pumping ability of the fuel pump. As shown in FIG. 11, the trochoid gear pump is composed of an outer gear 2 rotatably supported in an inner bore of a pump casing 1 and an inner gear 3 disposed in an eccentric relation to the outer gear 2. Pumping chambers 4 are formed between outer teeth of the inner gear 3 and inner teeth of the outer gear 2. The inner gear 3 is driven by a motor and the outer gear 2 rotates according to rotation of the inner gear 3, and thereby a volume of the pumping chambers is continuously changed. According to rotation of the trochoid gear pump, fuel is pumped up from a fuel tank and supplied to an internal combustion engine.
The fuel is sucked into the trochoid gear pump in a angular region where the volume of the pumping chambers 4 increases, and the sucked fuel is pressurized and pumped out in a angular region where the volume of the pumping chambers 4 decreases. Since the fuel pressure in the pumping chambers 4 increases in the region where the pumping chamber volume decreases, a load for outwardly pushing the outer gear 2 is generated in the pumping chambers 4. On the other hand, this load is not generated in the region where the pumping chamber volume increases. In other words, the load outwardly pushing the outer gear 2 only appears in the angular region where the fuel is pressurized. Accordingly, an inner imbalance load pressing the outer gear 2 against an inner bore of the pump casing 1 is generated in a direction shown by an arrow in FIG. 11. The outer periphery of the outer gear 2 is strongly pressed against the inner bore of the pump casing 1 by this inner imbalance load. This increases abrasion between the pump casing 1 and the outer gear 2, and thereby a higher driving torque is required. Accordingly, the pumping efficiency decreases and an operating life of the pump is shortened due to the abrasion.
To overcome the above-described problem, JP-A-2000-161241 proposes an improved structure of the trochoid gear pump. A relevant portion in the proposed structure is shown in FIG. 12. Fuel is sucked through an inlet port 5 and pressurized fuel in the pump is pumped out from an outlet port 6. A pressure-supplying groove 7 is formed on the inner bore of the pump casing 1 in the region where the fuel is pressurized and pumped out. The pressure-supplying groove 7 is connected to the outlet port 6 to introduce fuel pressure into the pressure-supplying groove 7. The outer periphery of the outer gear 2 is inwardly pushed by the fuel pressure introduced into the pressure-supplying groove 7. This proposed structure intends to cancel the inner imbalance load generated in the pumping chambers 4 by the pressure introduced into the pressure-supplying groove 7 and to decrease abrasion between the pump casing 1 and the outer gear 2.
It is found out, however, that the fuel pressure introduced into the pressure-supplying groove 7 does not impose a sufficient pressure on the outer periphery of the outer gear 2 to cancel the inner imbalance load generated in the pumping chambers 4. This is because the pressure in the pressure-supplying groove 7 leaks out through a clearance between the outer periphery of the outer gear 2 and the inner bore of the casing 1. Further, the pump efficiency decreases because the outlet pressure leaks toward the inlet port 5.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved trochoid gear pump in which the abrasion between the outer gear and the pump casing is surely decreased and thereby to realize a high efficiency and a high reliability of the trochoid gear pump.
A trochoid gear pump having an inner gear and an outer gear is used as a fluid pump such as a fuel pump for pumping up fuel from a fuel tank and supplying pressurized fuel to an internal combustion engine. The trochoid gear pump is driven by an electric motor, and the outer gear is rotatably supported in an inner bore of a cylindrical pump casing. In pumping operation of the trochoid gear pump, an inner imbalance load is generated in a pressuring angular region of the trochoid gear pump. The inner imbalance load is imposed on an inner periphery of the outer gear in a certain direction, pushing the outer gear against the inner bore of the pump casing.
Plural blade ditches are formed around a cylindrical outer periphery of the outer gear to generate an outer imbalance load which cancels at least a part of the inner imbalance load. Liquid such as fuel introduced into the blade ditches through a pressure-introducing port is pressurized in the blade ditches, and the pressurized fuel is released through a pressure-releasing port. Both the pressure-introducing port and the pressure-releasing port are positioned on the inner bore of the pump casing so that the outer imbalance load is generated at angular position substantially opposing against the inner imbalance load.
Preferably, the positions of both of the pressure-introducing port and the pressure-releasing port are selected so that the direction of the outer imbalance load exactly opposes to the direction of the inner imbalance load. Further, the blade ditches are so designed that an amount of the outer imbalance load generated in the blade ditches becomes equal to an amount of the inner imbalance load. In this manner, the inner imbalance load is fully canceled by the outer imbalance load, thereby reducing abrasion between the outer periphery of the outer gear and the inner bore of the pump casing and attaining a high pumping efficiency and a high reliability of the trochoid gear pump.
Passages for leading fluid to the pressure-introducing port and for releasing the pressurized fuel from the pressure-releasing port may be formed on side covers covering both sides of the inner and outer gears. The blade ditches may be formed in a zigzag arrangement around the outer periphery of the outer gear to make a volume of each blade ditch small thereby to reduce noises generated in a fluid flow.
According to the present invention, the inner imbalance load inherently developing in the trochoid pump is substantially canceled by the outer imbalance load generated by introducing fluid pressure to the outer periphery of the outer gear, and thereby a high efficiency and a high reliability of the trochoid gear pump are realized.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.